Introduction—Hyperlipidemia and Arteriosclerosis
Cardiovascular, cerebrovascular, and peripheral vascular diseases are responsible for a significant number of deaths annually in many industrialized countries. One of the most common pathological processes underlying these diseases is arteriosclerosis. Arteriosclerosis is characterized by lesions, which begin as localized fatty thickenings in the inner aspects of blood vessels supplying blood to the heart, brain, and other organs and tissues throughout the body. Over time, these atherosclerotic lesions may ulcerate, exposing fatty plaque deposits that may break away and embolize within the circulation. Atherosclerotic lesions obstruct the lumens of the affected blood vessels and often reduce the blood flow within the blood vessels, which may result in ischemia of the tissue supplied by the blood vessel. Embolization of atherosclerotic plaques may produce acute obstruction and ischemia in distal blood vessels. Such ischemia, whether prolonged or acute, may result in a heart attack or stroke from which the patient may or may not recover. Similar ischemia in an artery supplying an extremity may result in gangrene requiring amputation of the extremity.
For some time, the medical community has recognized the relationship between arteriosclerosis and levels of dietary lipid, serum cholesterol, and serum triglycerides within a patient's blood stream. Many epidemiological studies have been conducted revealing that the amount of serum cholesterol within a patient's blood stream is a significant predictor of coronary disease. Similarly, the medical community has recognized the relationship between hyperlipidemia and insulin resistance, which can lead to diabetes mellitus. Further, hyperlipidemia and arteriosclerosis have been identified as being related to other major health problems, such as obesity and hypertension.
Cholesterol Transport
Cholesterol circulating in the blood is carried by plasma lipoproteins that transport lipids throughout the blood. The plasma lipoproteins are classified in five types according to size: chylomicrons (which are largest in size and lowest in density), very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) (which are the smallest and most dense). These plasma lipoproteins exhibit differences in size, density, diameter, protein content, phospholipid content and triacylglycerol content, known to one of ordinary skill in the art. Of these, the low-density lipoprotein (LDL) and high-density lipoprotein (HDL) are primarily the major cholesterol carrier proteins. The protein component of LDL, the apolipoprotein-B (Apo B) and its products comprise the atherogenic elements. Elevated plasma LDL levels and reduced HDL levels are recognized as the primary cause of coronary disease because Apo B is in highest concentration in LDL particles and is not present in HDL particles. Apolipoprotein A-1 (Apo A-1) and apolipoprotein A-2 (Apo A-2) are found in HDL. Other apolipoproteins, such as Apo C and its subtypes (C-1, C-2 and C-3), Apo D and Apo E are also found in HDL. Apo C and Apo E are also observed in LDL particles.
Numerous major classes of HDL particles including HDL2b, HDL2a, HDL3a, HDL3b and HDL3c have been reported (Segrest et al., Curr. Opin. Lipidol. 11:105-115, 2000). Various forms of HDL particles have been described on the basis of electrophoretic mobility on agarose as two major populations, a major fraction with α-HDL mobility and a minor fraction with migration similar to VLDL (Barrans et al., Biochemica Biophysica Acta 1300; 73-85, 1996). This latter fraction has been called pre-β HDL and these particles are thought to be the most efficient HDL particle subclass for inducing cellular cholesterol efflux (Segrest et al., Curr. Opin. Lipidol. 11:105-115, 2000). The pre-β HDL particles have been further separated into pre-β1 HDL, pre-β2 HDL and pre-β3 HDL. These lipoprotein particles are comprised of Apo A-1, phospholipids and free cholesterol. The pre-β HDL particles are considered to be the first acceptors of cellular free cholesterol and are essential in eventually transferring free and esterified cholesterol to α-HDL (Barrans et al., Biochemica Biophysica Acta 1300; 73-85, 1996). Pre-β3 HDL particles may transfer cholesterol to α-HDL or be converted to α-HDL. These pre-β HDL particles have been characterized in terms of their charge, molecular mass (ranging from 40 kDa-420 kDa), size (Stoke's radius 4 nm-15 nm), shape (ellipsoidal, discoidal or spherical) and chemical composition (protein (including Apo A-1), free cholesterol, esterified cholesterol, phospholipids and the ratio of free cholesterol to phospholipids (see Barrans et al., Biochemica Biophysica Acta 1300; 73-85, 1996 for additional details)). HDL levels are inversely correlated with atherosclerosis and coronary artery disease. Accordingly, what is needed is a method to decrease or remove cholesterol from these various HDL particles, especially the pre-β HDL particles, so that they are available to remove additional cholesterol from cells.
Cholesterol is synthesized by the liver or obtained from dietary sources. LDL is responsible for transferring cholesterol from the liver to tissues at different sites in the body. However, if LDL collects on the arterial walls, it undergoes oxidation caused by oxygen free radicals liberated from the body's chemical processes and interacts deleteriously with the blood vessels. The modified LDL causes white blood cells in the immune system to gather at the arterial walls, forming a fatty substance called plaque and injuring cellular layers that line blood vessels. The modified oxidized LDL also reduces the level of nitric oxide, which is responsible for relaxing the blood vessels and thereby allowing the blood to flow freely. As this process continues, the arterial walls slowly constrict, resulting in hardening of the arteries and thereby reducing blood flow. The gradual build-up of plaque can result in blockage of a coronary vessel and ultimately in a heart attack.
In contrast to LDL, high plasma HDL levels are desirable because they play a major role in “reverse cholesterol transport”, where the excess cholesterol is transferred from tissue sites to the liver where it is catabolized and eliminated. Optimal total cholesterol levels are 200 mg/dl or below with a LDL cholesterol level of 160 mg/dl or below and a HDL-cholesterol level of 45 mg/dl for men and 50 mg/dl for women. Lower LDL levels are recommended for individuals with a history of elevated cholesterol, atherosclerosis or coronary artery disease.
Current Methods of Treatment
Hyperlipidemia may be treated by changing a patient's diet. However, diet as a primary mode of therapy requires a major effort on the part of patients, physicians, nutritionists, dietitians, and other health care professionals and thus undesirably taxes the resources of health professionals. Another negative aspect of this therapy is that its success does not rest exclusively on diet. Rather, success of dietary therapy depends upon a combination of social, psychological, economic, and behavioral factors. Thus, therapy based only on correcting flaws within a patient's diet is not always successful.
In instances when dietary modification has been unsuccessful, drug therapy has been used as an alternative. Such therapy has included use of commercially available hypolipidemic drugs administered alone or in combination with other therapies as a supplement to dietary control. These drugs, called statins, include natural statins, lovastatin, pravastatin, simvastatin, fluvastatin, atorvastatin, and cerivastatin. Statins are particularly effective for lowering LDL levels and are also effective in the reduction of triglycerides, apparently in direct proportion to their LDL-lowering effects. Statins raise HDL levels, but to a lesser extent than other anti-cholesterol drugs. Statins also increase nitric oxide, which, as described above, is reduced in the presence of oxidized LDL.
Bile acid resins, another drug therapy, work by binding with bile acid, a substance made by the liver using cholesterol as one of the primary manufacturing components. Because the drugs bind with bile acids in the digestive tract, they are then excreted with the feces rather than being absorbed into the body. The liver, as a result, must take more cholesterol from the circulation to continue constructing bile acids, resulting in an overall decrease in LDL levels.
Nicotinic acid, or niacin, is also known as vitamin B3. It is extremely effective in reducing triglyceride levels and raising HDL levels higher than any other anti-cholesterol drug. Nicotinic acid also lowers LDL-cholesterol.
Fibric acid derivatives, or fibrates, are used to lower triglyceride levels and increase HDL when other drugs ordinarily used for these purposes, such as niacin, are not effective.
Probucol lowers LDL-cholesterol levels, however, it also lowers HDL levels. It is generally used for certain genetic disorders that cause high cholesterol levels, or in cases where other cholesterol-lowering drugs are ineffective or cannot be used.
Hypolipidemic drugs have had varying degrees of success in reducing blood lipid; however, none of the hypolipidemic drugs successfully treats all types of hyperlipidemia. While some hypolipidemic drugs have been fairly successful, the medical community has not found any conclusive evidence that hypolipidemic drugs cause regression of atherosclerosis. In addition, all hypolipidemic drugs have undesirable side effects. As a result of the lack of success of dietary control, drug therapy and other therapies, atherosclerosis remains a major cause of death in many parts of the world.
New therapies have been used to reduce the amount of lipid in patients for whom drug and diet therapies were not sufficiently effective. For example, extracorporeal procedures like plasmapheresis and LDL-apheresis have been employed and are shown to be effective in lowering LDL.
Plasmapheresis therapy or plasma exchange therapy, involves replacing a patient's plasma with donor plasma or more usually a plasma protein fraction. Plasmapheresis is a process whereby the blood plasma is removed from blood cells by a cell separator. The separator works either by spinning the blood at high speed to separate the cells from the fluid or by passing the blood through a membrane with pores so small that only the fluid component of the blood can pass through. The cells are returned to the person undergoing treatment, while the plasma is discarded and replaced with other fluids.
This treatment has resulted in complications due to the introduction of foreign proteins and transmission of infectious diseases. Further, plasmapheresis has the disadvantage of non-selective removal of all serum proteins, such as VLDL, LDL, and HDL. Moreover, plasmapheresis can result in several side effects including allergic reactions in the form of fever, chills, and rash and possibly even anaphylaxis.
As described above, it is not desirable to remove HDL, which is secreted from both the liver and the intestine as nascent, disk-shaped particles that contain cholesterol and phospholipids. HDL is believed to play a role in reverse cholesterol transport, which is the process by which excess cholesterol is removed from tissues and transported to the liver for reuse or disposal in the bile.
In contrast to plasmapheresis, the LDL-apheresis procedure selectively removes Apo B containing cholesterol, such as LDL, while retaining HDL. Several methods for LDL-apheresis have been developed. These techniques include absorption of LDL in heparin-agarose beads, the use of immobilized LDL-antibodies, cascade filtration absorption to immobilize dextran sulphate, and LDL precipitation at low pH in the presence of heparin. Each method described above is effective in removing LDL. This treatment process has disadvantages, however, including the failure to positively affect HDL or to cause a metabolic shift that can enhance atherosclerosis and other cardiovascular diseases. LDL apheresis merely treats patients with severe hyperlipidemia.
Yet another method of achieving a reduction in plasma cholesterol in homozygous familial hypercholesterolemia, heterozygous familial hypercholesterolemia and patients with acquired hyperlipidemia is an extracorporeal lipid elimination process, referred to as cholesterol apheresis. In cholesterol apheresis, blood is withdrawn from a patient, the plasma is separated from the blood, and the plasma is mixed with a solvent mixture. The solvent mixture extracts lipids from the plasma. Thereafter, the delipidated plasma is recombined with the patient's blood cells and returned to the patient.
Conventional extracorporeal delipidation processes, however, are directed toward the concurrent delipidation of LDL and HDL. This process can have a number of disadvantages. Because LDL is more difficult to delipidate, extracorporeal systems are designed to subject body fluid volumes to substantial processing, possibly through multiple stage solvent exposure and extraction steps. Vigorous multi-stage solvent exposure and extraction can have several drawbacks. It may be difficult to remove a sufficient amount of solvents from the delipidated plasma in order for the delipidated plasma to be safely returned to a patient.
Hence, existing apheresis and extracorporeal systems for treatment of plasma constituents suffer from a number of disadvantages that limit their ability to be used in clinical applications. A need exists for improved systems, apparatuses and methods capable of removing lipids from blood components in order to provide treatments and preventative measures for cardiovascular diseases. What is also needed is a method to selectively remove lipid from HDL particles and thereby create modified HDL particles with increased capacity to accept cholesterol. What is also needed is a method to selectively remove lipid from HDL particles and thereby create modified HDL particles with increased capacity to accept cholesterol, without substantially affecting LDL particles.